(Adapted from the Applicant's Abstract) Abnormalities in formation of the conotruncal region of the heart are frequently observed birth defects. While much attention has focused on the role of the cardiac neural crest in these abnormalities, relatively little information exists about the potential contribution that other cell populations within the developing outflow tract might have in the pathology of conotruncal diseases. The investigators have recently demonstrated that NF-ATc1, a member of the Nuclear Factor of activated T Cells family of calcium dependent transcription factors which regulate cytokine gene expression in mature T cells, is preferentially expressed in the developing endocardium of the heart. Furthermore, the investigators have shown that inactivation of the gene by a targeted null mutation results in severe defects in semilunar valve formation and subsequent mid gestation demise secondary to congestive heart failure. Thus, the investigators propose that NFATc plays a critical role in a signal transduction process unique to semilunar valve formation and have designed a strategy to "dissect" this NFATc-1 dependent regulatory cascade. The investigators will first determine if inactivation of NFATc1 results in alterations in endothelial cell proliferation (BrdU- labeling) or variations in endocardial apoptosis (TUNEL assay) during critical periods of valve morphogenesis. In addition, the investigators will determine if the defects observed are the result of an endothelial cell autonomous abnormality by use of double knock ES cells in the development of chimeric embryos. The investigators also propose to characterize the physiological phenotype associated with perturbation of NFATc1 by in utero analysis of cardiac function with a high resolution 2D and Doppler ultrasound backscatter microscope. Finally, the investigators will use the NFATc null mutant mice in a cDNA substraction protocol to isolate novel targets of NFATc activation and will use a recently discovered NFATc transmembrane receptor expressed in the embryo to isolate new extracellular ligands that are capable of stimulating NFATc dependent processes during outflow tract morphogenesis. Upon completion of these experiments, the investigators will have not only determined the role of NFATc in conotruncal malformations, but they will have defined critical elements in a unique calcium dependent calcineurin mediated signal transduction pathway essential for normal endocardial, aortic and pulmonary valve ontogeny.